put a barracks (and training squad) at the entrance of each cavern for natural guardhouses
spears for big beasties. swords n axes n hammers for armored foes
if you are serious about military, training should begin at embark with two dwarves constantly sparring with weapon and shield. give them a year of this with maybe a break month or two, and they will have attained fair skill.
then train up another set of migrants using those two dwarves as captains. at this point i begin armor training. rinse and repeat this process to maximize training growth.
as far as sieges go, if you keep odds 1 to 1 your dwarves will likely prevail. 2 to 1 means fewer injuries. Anything less and you risk being overwhelmed, as dwarves struggle to fight more than two opponents at the same time. it can be very rewarding to split sieges into conquer-able portions
remember, armor is very heavy. it is a significant speed detriment to be in full plate
if you don't have time to train up armor user skill, consider a light equipment setup with weapons, shield, helm, mail shirt.
finally, this is pro tip, but if you have to fight a flame breathing beast or a dragon, a flooded battle room can help with extinguishing burning dwarves. however. . there is a consideration of steam generated.
I find the tradeoff is worth it since burning dwarves struggle to fight, and random piles of flame will block pathing.
So the strategy is to build a flooded arena of 2/3 or 3/3 water, and then force the beast fight in that arena.